PHYSIOLOGICAL CHEMISTRY. 



protein will decide this point, since the excreted nitrogen mul- 

 tiplied by 6.25 gives a measure of the food protein. The most 

 accurate method of reaching the value of the excreted nitrogen 

 is by Kjeldahl determinations on the urine and feces, but good 

 approximate results are secured by determination of urea alone, 

 it being remembered that about 90 per cent of the urinary 

 nitrogen appears in this form. 



Respiration Apparatus. To determine the volume of 

 oxygen inhaled and carbon dioxide given off the animal or 

 person under experiment is placed in a respiration chamber of 

 some kind. In a form of respiration chamber sometimes 

 used, an accurately measured volume of air with known con- 

 tent of moisture and carbon dioxide is forced through. The 

 air leaving the tight chamber is analyzed and the amount of 

 carbon dioxide, moisture and oxygen determined. This last 

 determination may be made directly, or the loss of oxygen by 

 the respiration of the person in the cage may be found by 

 calculation from this basis : The sum of all the factors con- 

 sumed, that is the food and the oxygen, plus the body weight, 

 must be balanced by the weight of the body at the end of the 

 experiment plus the various excreted matters. If A repre- 

 sents the body weight at the beginning of the test and A' the 

 body weight at the end of the test, Ox the oxygen consumed, 

 F the (dry) food consumed. Ex the total excreta by weight, 

 then 



In some of the recent forms of respiration apparatus, espe- 

 cially that of Atwater and Rosa, extremely accurate results 

 are possible in the determination of carbon dioxide and mois- 

 ture produced; but with increase in size of the apparatus a 

 direct determination of oxygen difference becomes more and 

 more difficult. 



In the Zuntz apparatus, which is often used for short experi- 

 ments on the gaseous excretion only, a peculiar mouthpiece is 



